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            <div><h4 class="title">
                <a name="asio.overview.model.executors"></a><a class="link" href="executors.html" title="Executors">Executors</a>
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    <p>
        Every asynchronous agent has an associated <span class="emphasis"><em>executor</em></span>.
        An agent's executor determines how the agent's completion handlers are
        queued and ultimately run.
    </p>
    <p>
        Example uses of executors include:
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            <li class="listitem">
                Coordinating a group of asynchronous agents that operate on shared
                data structures, ensuring that the agents' completion handlers never
                run concurrently<sup>[<a name="asio.overview.model.executors.f0"
                                         href="#ftn.asio.overview.model.executors.f0" class="footnote">5</a>]</sup>.
            </li>
            <li class="listitem">
                Ensuring that agents are run on specified execution resource (e.g.
                a CPU) that is proximal to data or an event source (e.g. a NIC).
            </li>
            <li class="listitem">
                Denoting a group of related agents, and so enabling dynamic thread
                pools to make smarter scheduling decisions (such as moving the agents
                between execution resources as a unit).
            </li>
            <li class="listitem">
                Queuing all completion handlers to run on a GUI application thread,
                so that they may safely update user interface elements.
            </li>
            <li class="listitem">
                Returning an asynchronous operation's default executor as-is, to run
                completion handlers as close as possible to the event that triggered
                the operation's completion.
            </li>
            <li class="listitem">
                Adapting an asynchronous operation's default executor, to run code
                before and after every completion handler, such as logging, user authorisation,
                or exception handling.
            </li>
            <li class="listitem">
                Specifying a priority for an asynchronous agent and its completion
                handlers.
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    <p>
        The asynchronous operations within an asynchronous agent use the agent's
        associated executor to:
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                Track the existence of the work that the asynchronous operation represents,
                while the operation is outstanding.
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                Enqueue the completion handler for execution on completion of an operation.
            </li>
            <li class="listitem">
                Ensure that completion handlers do not run re-entrantly, if doing so
                might lead to inadvertent recursion and stack overflow.
            </li>
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    <p>
        Thus, an asynchronous agent's associated executor represents a policy of
        how, where, and when the agent should run, specified as a cross-cutting
        concern to the code that makes up the agent.
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        <br>
        <hr width="100" align="left">
        <div class="footnote"><p><sup>[<a name="ftn.asio.overview.model.executors.f0"
                                          href="#asio.overview.model.executors.f0" class="para">5</a>] </sup>
            In Asio, this kind of executor is called a <a class="link" href="../core/strands.html"
                                                          title="Strands: Use Threads Without Explicit Locking">strand</a>.
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            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
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